Process for the purification of alkylene carbonate

ABSTRACT

This invention concerns a process to provide high purity alkylene carbonate though use of multiple distillations wherein the unused fractions are recycled to the reactor. The high purity alkylene carbonate may be further purified by use of carbon to produce electrochromic or photochromic grade alkylene carbonate by reducing its UV absorbance.

This application is a continuation-in-part of co-pending application Ser. No. 09/167,361, filed Oct. 7, 1998.

BACKGROUND OF INVENTION

This invention concerns a method for the production of alkylene carbonate, especially high purity alkylene carbonate.

Alkylene carbonates are well known materials that have been produced commercially for decades. Alkylene carbonate may be manufactured by a variety of methods. One such method is described in U.S. Pat. No. 2,773,070 (1956). Some applications of alkylene carbonate demand use of very high purity products. For example, when alkylene carbonates are used as solvents for electrolyte salts in lithium batteries, the alkylene carbonate preferably contain essentially no impurities (e.g., glycol less than 20 parts per million (“ppm”)) and very low water amounts (also less than 20 ppm). In the past, such purification was accomplished, for instance, by treatment by distillation; however, the impure streams from the distillation tower(s), which may constitute up to 50 percent of the effluent from the carbonate reactor, are typically considered useless by-products that are destroyed. The present inventors have recognized that a need exists to remedy this wasteful practice and to provide a more economical process. The present inventors have also recognized that a need exists for high purity alkylene carbonate on a commercial scale.

In addition, it is known that elemental carbon in various forms is suitable for use in contact treatment for removing impurities from a variety of fluid materials including air, and liquids such as water and organic liquids. Elemental carbon is known to exist in a variety of forms, including amorphous forms including soot, carbon black, charcoals, and lampblack. The macroscopic properties of these materials depend upon their particle size and surface area. Of these several forms, the charcoals, and particularly activated charcoal, find frequent employment in the treatment of the aforesaid substances. Activated charcoal is a porous, microcrystalline carbonaceous material having a surface area between about 500 to 1500 square meters of surface area per gram. It is presumed that the purification of various substances contacted with charcoals occurs through the mechanism of adsorption of various chemical species upon the surface of the charcoal.

Organic liquids including but not limited to alkylene carbonates and in particular propylene carbonate often contain undesirable impurities acquired either through the manufacturing process or from packaging and conveyance. It is known in the art that the use of pre-treated carbons, such as acid-washed carbon, can remove color bodies (impurities having a color that can be seen by the human eye) of many organic compounds.

On the other hand, however, alkylene carbonates having a very low ultraviolet light UV absorbance were until recently of little or no importance commercially. Recently, the alkylene carbonates having low UV absorbance may be used in applications where electrochromic or photochromic grade alkylene carbonate is needed. These optical applications require very low alkylene carbonate absorbances that are typically lower than those that can be achieved by typical distillation. The target UV absorbances are less than about 0.350 at 220 nanometers (nm), with less than about 0.310 nm being preferred, and less than about 0.930 at 215 nm, with less than about 0.910 being preferred. These values are in neat solution (not diluted). Hence, processes to produce carbonates having a UV absorbance near the detection limits of current analytical instrumentation are nonexistent. As used to herein, measurement of ultraviolet light (“UV”) absorbance refers to UV light having wavelengths in the 220-350 nanometer range. For instance, until recently, there was no need for electronics grade propylene carbonate.

The inventors have recognized that a need exists for processes to manufacture alkylene carbonates having very low UV absorbance. The inventors have discovered a process for treatment of alkylene carbonates with charcoals that have been treated which unexpectedly provides an alkylene carbonate that nears the current detection limits for UV absorbance.

SUMMARY OF INVENTION

The present invention provides a solution to one or more of the disadvantages and deficiencies described above.

In one broad respect, this invention is a process useful for the manufacture of alkylene carbonate, comprising: contacting carbon dioxide, an alkylene oxide, and a carbonation catalyst in a reaction zone to produce a crude reactor effluent; subjecting the crude reactor effluent to low temperature evaporation to form an evaporator overhead containing alkylene carbonate and an evaporator bottoms stream containing the catalyst, and recycling the evaporator bottoms stream to the reaction zone, removing any light components present in the evaporator overhead to form a second evaporator overhead and recycling the light components to the reaction zone; distilling the second evaporator overhead to form a first distillation overhead stream and a first distillation bottoms stream containing alkylene carbonate, and recycling the first distillation overhead stream to the reaction zone; distilling the first distillation bottoms stream to form a second distillation overhead stream and a second distillation bottoms stream and recycling the second distillation bottoms stream to the reaction zone; distilling the second distillation overhead stream to form a third distillation overhead stream and a third distillation bottoms stream and recycling the third distillation overhead stream to the reaction zone; distilling the third distillation bottoms stream to form a fourth distillation overhead stream containing purified alkylene carbonate and a fourth distillation bottoms stream, and recycling the fourth distillation bottoms stream to the reaction zone. Optionally, this embodiment may include contacting the purified alkylene carbonate with carbon to reduce the UV absorbance of the purified alkylene carbonate.

In another broad respect, this invention is a process useful for the manufacture of alkylene carbonate, comprising: distilling a first stream containing an alkylene carbonate in a purity of about 99 percent or more to form a first bottoms stream containing alkylene carbonate at a purity greater than the purification stream and an first overhead stream containing alkylene carbonate at a purity greater than the purification stream, and introducing the first overhead stream to an alkylene carbonate reactor; distilling the first bottoms stream to form a second overhead stream containing high purity alkylene carbonate and a second bottoms stream, and recycling the second bottoms stream to the alkylene carbonate reactor. Optionally, this embodiment may include contacting the purified alkylene carbonate with carbon to reduce the UV absorbance of the purified alkylene carbonate.

In another broad respect, this invention is process useful for the manufacture of alkylene carbonate, comprising: contacting carbon dioxide, an alkylene oxide, and a carbonation catalyst in a reactor to produce a crude reactor effluent; subjecting the crude reactor effluent to low temperature evaporation to form an evaporator overhead containing alkylene carbonate and an evaporator bottoms stream containing the catalyst, and recycling the evaporator bottoms stream to the reactor, removing any light components present in the evaporator overhead to form a second evaporator overhead and recycling the light components to the reactor; distilling the second evaporator overhead to form a first distillation overhead stream and a first distillation bottoms stream containing alkylene carbonate, and recycling the first distillation overhead stream to the reactor; distilling the first distillation bottoms stream to form a second distillation overhead stream and a second distillation bottoms stream and recycling the second distillation bottoms stream to the reactor; distilling the second distillation overhead stream in a distillation column to form a third distillation overhead stream, a high purity middle fraction having a purity of at least 99.99% and a third distillation bottoms stream, withdrawing the middle fraction from the column, and recycling the third distillation overhead stream and the third distillation bottoms stream to the reactor. Optionally, this embodiment may include contacting the purified alkylene carbonate with carbon to reduce the UV absorbance of the purified alkylene carbonate.

In yet another broad respect, this invention is a process useful for the manufacture of ethylene carbonate, comprising: contacting carbon dioxide, an ethylene oxide, and a carbonation catalyst in a reactor to produce a crude reactor effluent; subjecting the crude reactor effluent to low temperature evaporation to form an evaporator overhead containing ethylene carbonate and an evaporator bottoms stream containing the catalyst, and recycling the evaporator bottoms stream to the reactor, removing any light components present in the evaporator overhead to form a second evaporator overhead and recycling the light components to the reactor; subjecting the second evaporator overhead to a second low temperature evaporation to form a less pure fraction and a more pure fraction, and recycling the less pure fraction to the reactor; and either: (1) distilling the more pure fraction in a distillation column to form a less pure overhead fraction, a high purity middle fraction having a purity of at least 99.99% and a less pure bottoms fraction, withdrawing the middle fraction from the column, and recycling the less pure overhead fraction and the less pure bottoms fraction to the reactor, or (2) distilling the more pure fraction to form a distillation overhead stream and a distillation bottoms stream and recycling the distillation overhead stream to the reactor; distilling the distillation bottoms stream to form a second distillation overhead stream containing purified alkylene carbonate having a purity of at least 99.99% and a second distillation bottoms stream, and recycling the second distillation bottoms stream to the reactor. Optionally, this embodiment may include contacting the purified alkylene carbonate with carbon to reduce the UV absorbance of the purified alkylene carbonate.

In one broad respect, this invention is a process useful for the manufacture of alkylene carbonate, comprising: contacting carbon dioxide, an alkylene oxide, and a carbonation catalyst in a reaction zone to produce a crude reactor effluent; subjecting the crude reactor effluent to low temperature evaporation to form an evaporator overhead containing alkylene carbonate and an evaporator bottoms stream containing the catalyst, and recycling the evaporator bottoms stream to the reaction zone, removing any light components present in the evaporator overhead to form a second evaporator overhead and recycling the light components to the reaction zone; distilling the second evaporator overhead to form a first distillation overhead stream and a first distillation bottoms stream containing alkylene carbonate, and recycling the first distillation overhead stream to the reaction zone; distilling the first distillation bottoms stream to form a second distillation overhead stream and a second distillation bottoms stream and recycling the second distillation bottoms stream to the reaction zone; distilling the second distillation overhead stream to form a third distillation overhead stream and a third distillation bottoms stream and recycling the third distillation overhead stream to the reaction zone; distilling the third distillation bottoms stream to form a fourth distillation overhead stream containing purified alkylene carbonate and a fourth distillation bottoms stream, and recycling the fourth distillation bottoms stream to the reaction zone. This process may further comprise contacting the purified alkylene carbonate with carbon to reduce the UV absorbance of the purified alkylene carbonate.

In another broad respect, this invention is alkylene carbonate having a ultraviolet absorbance of less than 0.35 at 220 nanometers.

In another broad respect, this invention is a process useful for reducing the ultraviolet absorbance of an alkylene carbonate, comprising: contacting an alkylene carbonate with carbon under conditions effective to reduce the ultraviolet absorbance of the alkylene carbonate.

This process of contacting the alkylene carbonate with the carbon imparts superior UV light absorbance characteristics to the alkylene carbonate. Typically, the starting material for is colorless in the spectrum visible to the human eye. The carbon serves to remove colorless impurities that increase the UV absorbance of the starting alkylene carbon. The process employs a charcoal (carbon) that has been treated by contacting the charcoal with an inorganic acid, rinsing the charcoal with a solvent, and optionally drying the charcoal. The alkylene carbonate may be contacted with the so-treated charcoal to reduce the UV absorbance of the alkylene carbonate. The contacting may occur in a variety of ways, including contact in a packed column where the alkylene carbonate is flowed through the charcoal bed. The purified alkylene carbonate frequently a UV light absorbance at a wavelength of 220 nanometers (nm) of 1 or less, and an absorbance of 0.4 or less at 215 nm. This represents a substantial advantage over the prior art methods which, in the case of alkylene carbonates, is especially beneficial when the alkylene carbonate is to be employed as a raw material in the manufacture of products requiring ultra high purity alkylene carbonates.

This invention has a number of advantages. For example, high purity alkylene carbonate may be produced more cost-effectively as compared to existing practices. The process of this invention, furthermore, generates less waste and higher yields than existing processes. Advantageously, this process may be implemented using conventional equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a representative process scheme for the practice of this invention.

FIG. 2 shows another representative process scheme for the practice of this invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 there is shown a representative configuration for the practice of this invention. The starting reactants for production of alkylene carbonate, alkylene oxide and carbon dioxide, are introduced into the carbonate reactor 10 via line 12. It should be appreciated that while lines and conduits are depicted in FIG. 1, such lines and conduits need not be present and the effluents may be conveyed between apparatuses and method.

In accordance with this invention, alkylene oxides may be reacted in the reactor 10 with carbon dioxide in the presence of ammonium halides having the formula

NR₁R₂R₃R₄X

Where X is any halide ion, and R₁, R₂, R₃, and R₄ may each be hydrogen, alkyl, aryl, alkenyl, alkaryl, or aralkyl in any combination or in which any two of the radicals R₁, R₂, R₃ and R₄ may be interconnected to form with the basic nitrogen atom a ring of the pyridine, piperidine, pyrollidine, pyrroline, morpholine, or thiomorpholine series. In certain embodiments, the alkyl group may contain from 1 to 20 carbon atoms, the aryl group may be phenyl or naphthyl, the alkenyl group may contain from 2 to 20 carbon atoms, the alkaryl group may be an alkyl substituted phenyl or naphthyl in which the alkyl group may contain from 1 to 4 carbon atoms and the aralkyl group may be an alkyl group that may contain from 1 to 4 carbon atoms substituted by a phenyl or naphthyl radical.

The alkylene oxides which may be employed in the reaction are those of the oxirane system. Preferably, the alkylene oxides employed have a structural formula

in which W, Y, and Z may be hydrogen, or the groups alkyl containing from 1 to 20 carbon atoms, aryl containing from 6 to 12 carbon atoms, cycloalkyl containing from 5 to 20 carbon atoms, alkenyl containing from 2 to 20 carbon atoms, or in which any two of the groups W, Y, and Z may be interconnected to form with the two carbon atoms shown in the formula a carbocyclic ring. Ethylene oxide, propylene oxide and butylene oxide are representative examples of such alkylene oxides.

The reaction may be carried out at a temperature of from about 100 degrees Centigrade to about 225 degrees Centigrade, preferably from about 175 degrees Centigrade to about 215 degrees Centigrade, and under a pressure of more than about 300 pounds per square inch gauge, preferably from about 1,000 to about 3,000 pounds per square inch gauge. The reaction may be conducted either batchwise or continuously. For example, the catalyst may be continuously introduced in solution form along with the alkylene oxide and the carbon dioxide under desired pressure into one end of a reaction vessel and the products of reaction continuously withdrawn from the other end. A preferred solvent for the catalyst is the alkylene carbonate reaction product or a tertiary alcohol, e.g., tertiary butyl or amyl alcohol. Alternatively, batches of the alkylene oxide and the catalyst may be introduced into an autoclave or bomb type of reactor, the desired pressure built up by introducing carbon dioxide and the reaction mixture agitated while being heated to the reaction temperature and maintained under a superatomospheric pressure of carbon dioxide. Irrespective of whether a batch or continuous procedure is followed, each unit weight of reactants and reaction products resulting therefrom is maintained at reaction temperature and pressure for from about 1 to about 90 minutes, preferably from about 30 to about 60 minutes. This time interval is referred to herein as the reaction time.

The alkylene oxide and carbon dioxide are mixed in proportions to provide an excess of carbon dioxide over and above the stoichiometric amount required for reaction. This excess may be of the order of from 1% to 500% by weight.

The ammonium halide may be obtained as such from any available source or produced in any desired manner. While ammonium iodides, bromides, chlorides, and fluorides are all of them effective in catalyzing the synthesis of alkylene carbonates from alkylene oxides and CO₂, the iodides and bromides are generally considered to be more effective than the chlorides and fluorides. It is preferred to use the bromides since they are highly effective and in addition are much more stable under conditions of use than are the iodides, which tend to decompose on heating with evolution of elemental iodine which poses an additional purification problem. The ammonium radical may be unsubstituted (NH₄)⁺ or mono-, di-, tri-, or tetrasubstituted. Preferably, a tetrasubstituted ammonium halide is employed.

Representative examples of preferred catalysts include but are not limited to tetraethyl ammonium bromide, tetramethyl ammonium bromide, benzyltriethyl ammonium bromide and tetrabutyl ammonium bromide. These catalysts may easily be produced by heating a tertiary amine with an alkyl bromide. Thus, from triethyl amine and benzyl bromide, benzyltriethyl ammonium bromide is obtained. The ammonium halide catalysts may be purified by crystallization from a suitable solvent: in most cases an alcohol may be used for this purification. Methyl and ethyl alcohols are satisfactory for this purification in the case of most ammonium halides; however, a preferred solvent for tetraethylammonium bromide is tertiary butyl alcohol in which the catalyst is almost completely insoluble at room temperature, but in which it is quite soluble near the boiling point. Tertiary amyl alcohol is similarly well suited for this use.

The amount of catalyst used in general should be from 0.1% to 10%, preferably from about 1 to about 5% based on the weight of the reaction mixture. In general, the greater the catalyst concentration, within these limits, the more rapid and complete the reaction.

The carbonate reactor may be operated as described in U.S. Pat. No. 2,773,070 and W. J. Peppel, “Preparation and Properties of the Alkylene Carbonates,” Industrial and Engineering Chemistry, Volume 50, Number 5, May, 1958. The reactor 10 may be of conventional design as is currently being used in industry for this reaction.

The crude reactor effluent from reactor 10 may be conveyed via line 14 to an evaporation apparatus 20. The evaporation apparatus 20 may be of conventional design and is operated such that a low residence time is maintained to minimize degradation of the catalyst at high temperatures. The bottoms from the evaporator 20 contain inter alia, the catalyst. The overhead contains alkylene product and lights. The evaporator may be, for example, a wiped film evaporator or falling film tower. Typically, the evaporator is operated at a temperature from about 50 to about 150 degrees Centigrade, and at a pressure of form about 0.1 to about 100 mm Hg. If the catalyst is not sensitive to high temperatures, it may not be necessary to employ an evaporator having low residence time. The bottoms may be recycled to the reactor 10 via conduit 22. Typically, the evaporator splits the material such that about 5 to about 20 percent exit as bottoms, with about 80 to about 95 percent being overhead. The alkylene product stream exiting the evaporator usually has a purity in the range of about 98 to about 99.5 percent. Optionally, a second evaporator may be employed in series, again with the less pure fractions being returned to the reactor.

Optionally, the effluent from reactor 10 may be sent to a finishing drum, not shown. After removing lights from the evaporator overhead (using for instance a low pressure separator and/or a gas-liquid separator), the overhead is sent, directly or indirectly, to a first distillation tower 30 via line 24. The product may for example be sent to a storage unit prior to distillation. The first distillation tower, and all distillation towers used herein, serve to further purify the alkylene carbonate. The first distillation tower may be operated at any temperature and pressure which will afford a first distillation bottoms that is a higher purity than the alkylene carbonate received from the evaporator. In general, the first distillation tower is operated at a temperature of from about 50 degrees Centigrade to about 150 degrees Centigrade and a pressure of from about 0.1 to about 100 mm Hg.

The overhead from the first distillation tower may be recycled to reactor 10 via line 32. The first distillation bottoms, which constitutes about 90 to about 99 percent of the material fed to the first distillation tower, exits the first tower 30 via conduit 34, and is transferred to the second distillation tower 40.

In second distillation tower 40, the first distillation bottoms is subjected to additional purification. The second distillation bottoms may be recycled to the reactor 10 via conduit 42. The purified alkylene carbonate exits the second tower 40 via line 44. The purity of the alkylene carbonate stream exiting the second distillation column is usually in the range from about 99.5 to about 99.95 percent.

To achieve even further purification, the second distillation overhead is then subjected to two additional distillations. The additional distillations may be accomplished in a variety of ways. For example, the second distillation overhead may be stored and reintroduced into first distillation tower 30 via line 28. This would be done when the reactor 10 and evaporator 20 were not running. The overhead from the first distillation tower 30 and bottoms from the second distillation tower 40 would again be recycled to reactor 10. This recycling provides many advantages. The most important advantages are conservation of mass, which provides a high overall yield, and a cost advantage as contrasted against processes where such overhead and bottoms are destroyed or not used to make additional high purity alkylene carbonate.

In another alternative, the second distillation overhead is sent to another tower or towers different from the towers 30 and 40 shown in FIG. 1. For example, the overhead may be sent to a single, very large tower instead of two smaller towers in series. The larger tower may have 50 to 150 theoretical plates containing for instance 100 trays and packing, as opposed to smaller towers having 40 to 60 trays. In this case, the middle fraction from the large tower is the high purity alkylene carbonate, with the overhead and bottoms being recycled to the reactor 10. Hence, recycling of fractions to the reactor 10 would still be performed even if a single tower were used or if the effluent was sent to other towers, off-site or otherwise, for further purification. It should be appreciated that an important aspect of this invention is the return of the less pure fractions to the reactor, which leads to higher yields, less waste and a more economical high purity alkylene carbonate process.

Still another alternative is depicted in FIG. 2. In this generalized scheme, four towers are used in series. FIG. 2 is identical to FIG. 1 except additional columns 50 and 60 are included. Instead of sending product effluent from second distillation tower 40 to first distillation tower 30 or to a separate distillation tower or towers, the effluent flows into the third distillation tower 50 via line 44. The overhead from the third distillation tower 50 is recycled to reactor 10. The third distillation bottoms is introduced into fourth distillation tower 60 via line 54. The fourth distillation bottoms is recycled to line 12 and reactor 10 via conduit 62. The final alkylene carbonate product exits the fourth distillation tower via line 64.

In general, the distillation towers (also referred to as columns) may be of conventional design. The towers may be packed with conventional packing. The temperature and pressure in the tower may be adjusted depending on the type of alkylene carbonate being produced. In general, particularly for ethylene carbonate and propylene carbonate, the tower is maintained at a temperature in the range from about 50 degrees Centigrade to about 150 degrees Centigrade Centigrade, and the pressure is in the range from about 0.1 to about 100 mm Hg.

Prior to carbon treatment, the alkylene product produced by the process of this invention has a purity of at least 99.99 percent. Typically the final alkylene carbonate has a purity up to 100 percent and more typically more than about 99.999 percent. The final product typically has a water content less than about 20 parts per million (“ppm”) and impurity levels less than 20 ppm.

It should also be appreciated that the alkylene carbonate may be made in the reactor from a variety of methods, such as from ethylene glycol and phosgene such as described in Neminowsky, J. Prakt. Chem., [2] 28, 3789 (1955); from diethyl carbonate and ethylene glycol by transesterification as described in Morgan et al., J. Am. Chem. Soc., 75, 1263 (1053); from ethylene chlorohydrin and sodium bicarbonate as described in U.S. Pat. No. 1,907,891; or from 1,2-epoxides and carbon dioxide as described in German patent 740,366 (1943).

The process of this invention, including each sub-step of the overall process, may be operated continuously, intermittently, or as a batch process.

Reduction of the Alkylene Carbonate UV Absorbance

Typically, the alkylene carbonate to be treated is colorless to the eye. The alkylene carbonate may, however, contain impurities that creates UV absorbance that is too high for some applications. To facilitate additional purification of the alkylene carbonate to thereby reduce its UV absorbance, the alkylene carbonate may be treated with carbon. For example, the effluent from line 44 from FIG. 1 or from line 64 in FIG. 2 may be pumped downwardly through a column packed with a fixed bed of carbon maintained at about 25 degrees Centigrade and at a space velocity (flow rate) of about 5-10 mL of alkylene carbonate/250 mL of carbon/hour. In one embodiment, this may be achieved through use of a one inch diameter (e.g., a stainless steel pipe) that has been packed with 250 mL of carbon. Then, alkylene carbonate may be added to the column and optionally heated to 50 degrees Centigrade and held overnight, with the alkylene carbonate then being drained and alkylene carbonate to be treated thereafter added to the column. Typically, the alkylene carbonate to be treated is of high purity and contains less than 20 parts per million (ppm) of water and less than 20 ppm of alkylene glycol impurities (such as propylene glycol in the case of propylene carbonate). The carbon and the carbon treatment will now be described.

The carbon useful in this process can be any conventional carbon or charcoal used as an absorbent. Carbon, activated carbon and charcoal are widely available commercially. Carbon can be rendered active using conventional procedures such as treatment with dilute aqueous hydrogen chloride. Suitable carbon can be used from a wide variety of sources. For example, carbon known as bituminous coal type and coconut shell type are well known in the art. The shape of the carbon is not critical and can be in the form of any conventional shape such as powder, granular, pellet, or the like. In one embodiment, charcoal is employed and is generally preferred in the amorphous form of carbon for use in accordance hereto, and charcoal derived from coconut shells is especially preferred. The surface area of the carbon can vary widely from about 500 to 1500 m²/g. The average size of carbon used in this invention can vary widely, but finely powdered carbons are less desirable since they are difficult to separate from the alkylene carbonate and tend to cause plugging in a conventional continuous flow system. Any size carbon can be used which is capable of being supported in a bed without plugging, as is apparent to a skilled artisan. A representative example of a suitable carbon is 12×30 mesh Calgon™ 300 Gly which is commercially available from Calgon Carbon Corporation, and which is believed by the inventors to have been washed with an inorganic acid.

The carbon may be pre-treated by contacting it with an inorganic acid prior to being the alkylene carbonate. For this purpose, any inorganic acid, or its solutions in water or organic solvents may be used. Suitable organic solvents include solvents in which the acid is miscible, such as alcohols and ethers which are readily removed by drying. Subsequent to being contacted with an inorganic acid, the carbon employed may be rinsed with either deionized water, then organic solvent, such as methanol. Following rinsing, the carbon is preferably dried (typically by heating the carbon), then rinsed with alkylene carbonate prior to using the carbon in the treatment process.

The treatment process using carbon can be run either batch-wise or in a continuous manner.

In a batch mode, the alkylene carbonate is contacted with the carbon in a closed vessel for a time sufficient to remove at least a portion of the color therein. The process of this invention can be conducted in any batch system suitably designed for such purpose as is apparent to a skilled artisan. Contact time will vary depending on factors such as temperature, pressure, volume of alkylene to be treated, and the amount of alkylene carbonate relative to carbon. Typically, contact time is greater than 0.1 hour. Preferably, time is greater than 0.5 hour. Typically, time is less than 24 hours. Preferably, time is less than 18 hours, more preferably less than 8 hours. In batch mode, the amount of carbon is at least one percent by weight relative to alkylene carbonate. The amount of carbon is preferably greater than about 5 percent. The batch can be stirred. Pressure can be atmospheric, sub-atmospheric, or super-atmospheric. A pad of an inert gas such as dry nitrogen can be maintained over the batch. After treatment, the alkylene carbonate can be separated from carbon using conventional techniques such as filtration.

In a continuous treatment process, the alkylene carbonate to be treated is contacted with one or more fixed beds of carbon. Conventional treatment apparatus are useful for this purpose. The contact time varies depending on conditions and may be expressed in terms of flow rate over carbon. Typically, the flow rate is greater than about 0.1 mL of alkylene carbonate mL per 250 mL of carbon per hour, and in one embodiment greater than about 1 mL of alkylene carbonate per 250 mL carbon per hour. Typically, the flow rate is usually no more than about 50 mL of alkylene carbonate per 250 mL of carbon per hour, in one embodiment the flow rate is usually no more than about 50 mL of alkylene carbonate per 250 mL of carbon per hour, and in another embodiment is about 5 mL of alkylene carbonate per 250 mL carbon per hour. Pressures are preferably sufficient to maintain liquid conditions. In continuous operation, the apparatus is usually equipped in a conventional manner so that effluent is free of carbon particles.

In any mode by which the process of this carbon treatment is conducted, temperature is typically greater than or equal to about 0 degree Centigrade, and more frequently greater than or equal to about 10 degrees Centigrade. Preferably, temperature is typically less than or equal to about 200 degrees Centigrade, more typically less than or equal to about 100 degrees Centigrade, more typically less than or equal to about 50 degrees Centigrade. In one embodiment, the temperature is in the range from about 10 to about 50 degrees Centigrade. In another embodiment, the temperature is in the range from about 20 to about 30 degrees Centigrade. This is advantageous because the treatment can occur under ambient temperatures. In the case of ethylene carbonate, depending on pressure, the temperature should typically be 40 degrees Centigrade or higher to ensure that the ethylene carbonate does not solidify.

In any mode by which the process of this carbon treatment is conducted, the process is conducted under conditions effective such that the UV absorbance of the resulting alkylene carbonate is typically less than about 0.350 at 220 nanometers (nm), with less than about 0.310 nm being preferred, and typically less than about 0.930 at 215 nm, with less than about 0.910 being preferred.

The alkylene carbonate made according to this invention is also advantageously low in water content (for example, less than about 20 ppm) and low in alkylene glycol by-product (less than about 20 ppm), particularly for the propylene carbonate.

The activity of the carbon may decline over time. Therefore, the carbon may require regeneration as necessary as determined by routine experimentation and observation. Conventional procedures can be employed for this purpose. A polar solvent may be used to flush the carbon and thereby remove adsorbed color bodies. Likewise, the carbon can be heated to burn off deposits.

The following examples are illustrative of this invention are not to be construed to limit the scope of the instant invention or claims hereto. Unless otherwise denoted, all percentages are by weight.

Propylene carbonate obtained from a process as depicted in FIG. 2 was employed in this example. The propylene carbonate had a purity of greater than 99.99% and a UV absorbance of 0.355 at 220 nm and 0.940 at 215. A column containing a carbon bed (133.5 grams of Calgon™ 300 Gly, 12×30 mesh, which corresponded to 250 mL of carbon) was flushed with propylene carbonate at a rate of 5 mL of alkylene carbonate/250 mL of carbon/hour until the column was full. Then, the filled column was heated to 50 degrees Centigrade and held at that temperature while standing overnight. The propylene carbonate was then drained from the column. Propylene carbonate was then pumped with a downwardly flow rate of 5 or 10 mL/250 mL of carbon/hour. Several runs were made. The results are shown in Tables 1 and 2. The propylene bonate temperature is shown for each run. In the examples, the temperature was not increased because as temperature increases, the driving force for impurity absorption is decreased. In each one quart of purified propylene carbonate was collected and tested for UV absorbance.

TABLE 1 FLOW RATE OF 10 ML CARBONATE/ML CARBON/HOUR UV Run time Temper- Absorbance Num- Cut-off Cut ature Feed Rate at ber Number for a cut (° C.) (mL/mL/hr) 220/215 nm 1 start  9:30 am 30.0 10  0.35/0.940 treatment 1 1 10:45 am 25.0 10 0.281/0.860 1 2 11:45 am 24.9 10 0.265/0.929 1 3 12:45 pm 25.1 10 0.278/0.849 1 4  1:45 pm 24.6 10 0.321/0.919 2 5  6:45 am 25.6 10 0.259/0.815 2 6  8:45 am 25.1 10 0.295/0.872 2 7 10:45 am 25.0 10 0.310/0.899 2 8 12:45 pm 24.9 10 0.293/0.878 3 9  8:45 am 25.6 10 0.299/0.891 3 10 10:45 am 25.0 10 0.303/0.899 3 11 12:45pm 25.0 10 0.286/0.867 4 12  7:45 am 25.3 10 0.275/0.852 5 13  8:30 am 25.5 10 0.284/0.867 5 14 10:30 am 25.1 10 0.305/0.898

TABLE 2 FLOW RATE OF 5 ML CARBONATE/ML CARBON/HOUR UV Run time Temper- Absorbance Num- Cut-off Cut ature Feed Rate at ber Number for a cut (° C.) (mL/mL/hr) 220/215 nm 6 16  7:26 am 25.2 5 0.305/0.898 6 17  9:31 am 24.7 5 0.279/0.858 6 18 11:30 am 24.9 5 0.294/0.885 6 19  2:30 pm 25.1 5 0.299/0.688 7 20  7:35 am 28.2 5 0.308/0.901 7 21  9:35 am 25.0 5 0.277/0.834 7 22 11:35 am 25.0 5 0.273/0.832 7 23  2:33 pm 25.2 5 0.280/0.844 8 24  7:45 am 25.5 5 0.286/0.851 8 25  9:45 am 25.1 5 0.292/0.860 9 26  8:21 am 25.2 5 0.322/0.945 9 27 10:22 am 25.0 5 0.318/0.912 9 28 12:30 pm 25.1 5 0.302/0.895 10 29  7:53 am 25.5 5 0.289/0.885 10 30  9:52 am 25.0 5 0.296/0.867 10 31 12:15 pm 25.0 5 0.315/0.904 10 32  2:03 pm 25.0 5 0.325/0.904 11 33  8:30 am 25.3 5 0.307/0.887 11 34 10:30 am 25.0 5 0.322/0.902 11 35 12:30 pm 25.2 5 0.304/0.885 12 36  8:20 am 25.6 5 0.307/0.888 12 37 10:23 am 25.0 5 0.298/0.874 12 38 12:25 pm 25.0 5 0.295/0.862 13 39  7:00 am 25.3 5 0.317/0.902 13 40  9:03 am 25.0 5 0.318/0.903 13 41 11:08 am 25.1 5 0.336/0.922 13 42  1:15 pm 25.1 5 0.319/0.902 14 43  8:07 am 25.6 5 0.315/0.897 14 44 10:12 am 25.0 5 0.303/0.883 15 45  7:15 am 25.7 5 0.318/0.905 15 46  9:00 am 25.1 5 0.305/0.891 15 47 11:05 am 25.0 5 0.314/0.897 15 48  2:15 pm 24.9 5 0.316/0.889

These examples illustrate that carbon provides a surprisingly low UV absorbance of alkylene carbonates such as propylene carbonate. While final UV absorbances varied from run to run, in all runs the UV absorbance was reduced from the starting point of 0.35 at 220 nm, thereby producing alkylene carbonate having a UV absorbance typically suitable for electrochromic or photochromic applications. Similarly, with respect to 215 nm UV light, in all runs the UV absorbance was reduced from the starting point of 0.94, thereby producing alkylene carbonate having a UV absorbance typically suitable for electrochromic or photochromic applications. While it has been known to remove color bodies from organic liquids such as alkylene carbonates, it has been heretofore unknown to decrease the UV absorbance of such materials, particularly alkylene carbonate such as propylene carbonate.

Further modifications and alternative embodiments of this invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the invention. It is to be understood that the forms of the invention herein shown and described are to be taken as illustrative embodiments. Equivalent elements or materials may be substituted for those illustrated and described herein, and certain features of the invention may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. 

What is claimed is:
 1. A process useful for reducing the ultraviolet absorbance of an alkylene carbonate, comprising: contacting an alkylene carbonate with carbon under conditions effective to reduce the ultraviolet absorbance of the alkylene carbonate.
 2. The process of claim 1 wherein the alkylene carbonate is ethylene carbonate, propylene carbonate, or butylene carbonate.
 3. The process of claim 1 wherein the alkylene carbonate is propylene carbonate.
 4. The process of claim 1 wherein prior to contact with the carbon the alkylene carbonate has a purity of at least about 99.99 percent, a water content less than about 20 parts per million, a glycol content of less than 20 parts per million, and a ultraviolet absorbance of more than 0.350 at 220 nanometers.
 5. The process of claim 1 wherein the alkylene carbonate is contacted with the carbon at a temperature in the range of from about 10 degrees Centigrade to about 50 degrees Centigrade.
 6. The process of claim 1 wherein the purified alkylene carbonate is contacted with the carbon in a continuous manner and at a flow rate in the range of from about 1 to about 50 mL of alkylene carbonate per mL of carbon per hour.
 7. The process of claim 1 wherein the carbon has been pre-treated with an inorganic acid. 